Trucks include so-called van-type trucks which, as shown in FIG. 15, have a cargo box 1 of the sealed kind for the purpose of protecting cargo from rain, wind and dust and to prevent loss and theft of the cargo. However, in order to load and unload cargo, it goes without saying that a large opening is required.
Conventionally, a rear door of the kind shown in FIGS. 16, 17 [in both of which (A) is a side view and (B) a front view] is used for this purpose. More specifically, in FIG. 16, a rear door 2 of the cargo box 1 is composed of an upper door 2a and a lower 2b that are separate of each other. The upper door 2a is opened and closed by being rotated about an upper shaft 3a, and the lower door 3b is opened and closed by being rotated about a lower shaft 3b.
In FIG. 17, a rear door 2 constitutes a single body and is opened and closed by being rotated about a shaft 3.
In the view of the inventor, however, a rear door of this conventional type is disadvantageous in that a large moment is produced about the shaft, owing to the door's own weight, during the operation for opening and closing the rear door, thus making it difficult for a large-size door to be opened and closed by human power.
A rear door suspension apparatus of the kind shown in FIG. 18 [in which (A) is a front view and (B) a side view] has been proposed heretofore. In this case an air damper 4 is fitted between the rear door 2 and a cargo box body 1a in the manner illustrated. A gas such as compressed air is sealed within a cylinder 4a of the air damper 4. The moment produced by the door's own weight during the opening-and-closing operation is supported by the pressure of the gas, thus making it easy to open and close the door by human power.
In the view of the inventor, however, a drawback in the case of this proposed apparatus is that a sealing portion 4b of the cylinder sustains wear and allows the interior gas to leak out when the door is opened and closed frequently. Further, if cargo 5 is large in size, the air damper 4 can be damaged by the cargo, as indicated at arrow P, when the cargo is handled, and this can lead to leakage of the gas.